Electronic thermostats can be used to control the temperature in a building space as part of the building's heating, ventilation and air conditioning (“HVAC”) system. Thermostats typically receive temperature information from one or more temperature sensors. While simple thermostats react to the sensor inputs using only analog circuits, most modern thermostats run a microcontroller program or algorithm. The algorithm determines when the various devices controlled by the thermostat should be turned on or off based on the temperature data. Thus, thermostats function as switches to control devices such as furnaces, blowers, motors, and compressors. Rather than switching the full device load current, the controlled devices are typically switched through intermediate controls such as relays or solid state switches.
A thermostat is typically placed in or near the space where it is to control the temperature. A minimal number of wires are run from the thermostat's location to the location or locations of the various devices controlled by the thermostat. The most common configuration is to run a single control wire for each device with a common return wire for all of the device controls. Typically the control power for this circuit is 24 VAC provided by a control transformer.
Thermostat electronics circuitry can be powered by a local power source such as a battery. In addition, parts of the control circuitry can be powered by trickling some small amount of current from the control circuit using “power stealing”, that is drawing a relatively small amount of power from a device control line without actually switching the device on. One problem is to minimize the amount of power stealing so as to avoid a false switching of the device being controlled by that line. Another problem is that even with power stealing, there can still be significant battery drain by the output electronic circuits in the thermostat that controls the switching of each device controlled by the thermostat.
Accordingly there is a need for an electronic thermostat circuit topology that can minimize the current needed to control thermostat outputs.